Fourier transform optical tomographic imaging scanners use optical heterodyne techniques to observe the Fourier transform of a cross-section of a sample. Such scanners are useful for many applications where it is desirable to determine shape information of structures on or within samples. Optical tomographic scanners can be used for measuring features on opaque two-dimensional surfaces. For example, optical tomographic scanners can be used to determine fabrication tolerances in semiconductor devices. Optical tomographic scanners can also be used for measuring three dimensional shape information within a translucent sample. The shape information may be the location of fluorescent dye molecules, light absorbing features or scattering centers in the sample. For example, optical tomographic scanners can be used to produce volumetric images of biological samples.
In U.S. Pat. No. 4,584,484, a microscope is disclosed that exposes a localized sample to a moving sinusoidal interference fringe pattern that is produced by the interference of wave energy directed at the sample. Incident coherent light is directed at a sample and at a mirror. A resulting fringe pattern is produced from the interference between the reflected light and the incident light. The spatial frequency of fringes in the fringe pattern is a direct function of the angle of the incident light with respect to the sample. The spatial frequency varies between a first and second value during the motion or passage of the fringe pattern over the sample in a first direction.
The fringe patterns are passed over the sample multiple times. During each pass of the moving fringe pattern over the sample, a set of signals are recorded that are proportional to the intensity of the wave energy transmitted from the sample (by reflection or transmission). The direction of relative motion between the image of the moving fringe pattern and the sample is slightly changed by rotating the sample or by rotating the beam. Another set of signals is then recorded that are proportional to the intensity of the wave energy transmitted from the sample.
This process is repeated until "Fourier slices" in a considerable number of directions are generated and recorded. The recorded data is thereafter processed by extracting phase and amplitude values of at least some of the Fourier components of the recorded signals. These Fourier components contain information equivalent to the Fourier components of the image of the physical specimen measured at high resolution. The optical image of the sample or scene under examination is reconstructed by performing Fourier transforms on the Fourier components.
Prior art optical scanning instruments require the use of at least one degree of mechanical rotation that must be stepped through a large number of pairs of angular positions. This results in relatively long measurement acquisition times.